Two Detached ADUs in Castro Valley (AB 956, 2027)
Plan Two ADUs in Castro Valley: Free Site Assessment
Castro Valley ADU rules today (2026)
From January 1, 2027, state law (AB 956) requires counties, including Alameda County, to approve up to two detached ADUs on a lot with a single-family home. Castro Valley is unincorporated, so Alameda County issues ADU permits there, and the County has been revising its ADU ordinance after a state review. Here is where things stand.
- Ordinance: Alameda County Ordinance O-2024-32 (September 19, 2024) added Chapter 17.55 for ADUs and JADUs. Amendments responding to HCD went to the Planning Commission on December 1, 2025. The points below follow those amendments.
- Units allowed now: one JADU plus one new-construction ADU plus one conversion ADU on a single-family lot.
- Sizes: detached up to 850 sq ft (studio or one bedroom) or 1,200 sq ft (two or more bedrooms). Attached up to 50% of the main house, with 850 and 1,000 sq ft minimum allowances. JADUs up to 500 sq ft.
- Height: 16 ft for a detached ADU; 18 ft near transit; up to 20 ft to match roof pitch. Attached: 25 ft or the district limit. Two stories maximum.
- Setbacks: 4 ft side, street-side and rear. ADUs over 800 sq ft match the main house’s front setback.
- Separation: 6 ft from other buildings for ADUs over 800 sq ft.
- Parking: one space per ADU, allowed in setbacks or tandem, with exemptions near transit, for conversions and for studios, among others. None for a JADU.
- Process: ministerial, no hearing. ADUs over 800 sq ft follow the West County Residential Design Standards.
State review of Alameda County's ADU rules
On April 30, 2025, HCD found 39 points where the County’s 2024 ordinance did not comply with state law. They covered recent state ADU bills, floor-area limits, parking, an older 1998 second-unit policy, design standards, an 800 sq ft cap on detached units where a JADU exists, and the lack of a preapproved plan program. The County’s December 2025 amendments respond to these findings. Where local rules are stricter than state law, state law applies.
Recent Axiom work
Professional photos from a recent Axiom Renovation full remodel and addition in Los Gatos.
What changes on January 1, 2027
AB 956 raises the number of detached ADUs Alameda County must approve on a single-family lot from one to two, both new construction. Each may be limited to 800 sq ft of living space, local height limits can still apply, and side and rear setbacks can be no more than 4 ft. Where two detached ADUs are built, a JADU on the same lot does not have to be approved. The law also includes protections for owners in homeowners associations.
In Castro Valley today, only one ADU can be new construction; the second must be a conversion. AB 956 allows two new detached units. The County’s detached sizes of 850 and 1,200 sq ft are above the 800 sq ft each unit may be held to in a two-unit project. Units of 800 sq ft or less also avoid the 6 ft separation and the West County design standards that apply to larger ADUs. Alameda County will need to update its ordinance again, and we will confirm the current rules as it does.
Planning two detached ADUs on a Castro Valley lot
- Parking: unlike many cities, the County can require one space per ADU. Check whether the lot is within a half mile of transit or qualifies for another exemption.
- Size and spacing: keeping each unit at 800 sq ft or less avoids the 6 ft separation rule and the larger-ADU front setback.
- Electrical: two ADUs usually need a load calculation and often a service upgrade.
- Use and sale: owner occupancy applies only to a JADU. Rentals must be longer than 30 days, and separate sale is allowed only under Gov. Code §66341.
Building ADUs in Castro Valley with Axiom
See our Castro Valley ADU builder page and our Castro Valley service area, read the AB 956 overview, or compare nearby Hayward and Dublin.
Process
Our ADU Process
Step 1
Initial Consultation
We begin with a thorough initial consultation to understand your needs, assess your space, and discuss your vision, budget, and timeline.
Step 2
Planning and Design
At Axiom Renovation, we understand that the planning and design phase of an ADU project is crucial to its success. Our dedicated project managers will work closely with you to understand your goals, design ideas and budget for the project.
Step 3
Material Selection
We guide you through selecting high-quality materials that fit your design and budget, ensuring durability and aesthetic appeal.
Step 4
Demolition and Preparation
Careful demolition and preparation work are performed to ready the space for construction, minimizing disruptions and ensuring a clean work environment.
Step 5
Construction and Installation
Our skilled craftsmen handle the construction phase, including obtaining permits, scheduling appointments, and the actual construction and installation with precision, building the new space according to the approved design.
Step 6
Electrical and Plumbing Work
Licensed professionals conduct all electrical and plumbing work, ensuring everything is up to code and seamlessly integrated into the new ADU.
Step 7
Painting and Finishing
We pay close attention to detail during the painting and finishing stage, ensuring all surfaces are properly prepared and beautifully finished.
Step 8
Fixture and Appliance Installation
We expertly install all fixtures and appliances, ensuring they function correctly and fit perfectly with the overall design.
Step 9
Final Touches and Clean-Up
Final touches are added, and a thorough clean-up is performed, leaving your new space pristine and ready for use.
Step 10
Final Walkthrough and Client Satisfaction
We conduct a final walkthrough with you to ensure every detail meets your expectations and address any remaining concerns, ensuring your complete satisfaction.
Castro Valley Two-ADU FAQ
Can I build two detached ADUs in Castro Valley?
From January 1, 2027, AB 956 requires Alameda County, which issues permits in Castro Valley, to approve up to two detached, new-construction ADUs on a single-family lot that meets objective standards. Today one new ADU plus one conversion ADU and a JADU are allowed.
How large can each ADU be in Castro Valley?
Under the County’s current amendments, a detached ADU can be 850 sq ft with up to one bedroom or 1,200 sq ft with two or more. Under AB 956, the County may limit each of two detached ADUs to 800 sq ft of living space.
How tall can a detached ADU be in Castro Valley?
16 ft, or 18 ft near transit, and up to 20 ft to match the roof pitch of the house. Two stories is the maximum.
Does an ADU in Castro Valley need parking?
Often yes. The County requires one space per ADU, which can be tandem or in setbacks. Exemptions include lots within a half mile of transit, conversions and studios. A JADU needs no parking.
Who issues ADU permits in Castro Valley?
Alameda County. Castro Valley is unincorporated, so County ADU rules and County permits apply. The review is ministerial, with no public hearing.
Who builds two-ADU projects in Castro Valley?
Axiom Renovation designs, permits and builds ADUs in Castro Valley and is planning two-unit projects for 2027. Call 408-702-2022, Monday to Friday, 9 am to 5 pm.
Sources: AB 956 (Quirk-Silva), signed September 29, 2026, effective January 1, 2027, as summarized by Brownstein Hyatt Farber Schreck and California YIMBY; Alameda County ADU ordinance O-2024-32; HCD findings, April 30, 2025; Alameda County ADU Resource Center. This page is general information, not legal advice. Local rules change; we confirm current requirements with the County for every project.